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Tail / Stabilizer buffeting

Bugs66

Bugs66

Registered User
Spokane WA
I have around 20 hours on my experimental now and love flying the plane. One area I am focusing on now is stablizer buffeting. During landing approach when I pop my 2nd of 3 notch of flaps, I feel aerodynamic buffeting. My approaches are usually 55-60mph for routine landings on pavement. I would classify it as light buffeting. The final notch of flaps increases the buffeting slightly. The buffeting is coming from the stablizer because when I look back at the stabs I can see the buffet. When I get to flare the buffet disappears. At no time do I feel I don't have elevator authority. I have VG's on wing and under tail.

I have read some of the older posts of extended flaps causing buffeting. I have extended however not all the way to the fuselage and have about a 3" fairing.

Here are some other considerations:
- I have 84" prop
- 180hp with 0 degree thrust.
- I don't run flap gap seals
- I don't have tail shake on the ground, nor in climb or level flight.

My tail brace wires feel pretty good but might tighten a bit.

I would be curious and appreciate any comments anyone has with similar setups (extended flaps, etc.). I read that perhaps the BLR strakes might help in this area. Looking for a remedy.
 
We have a PA-12 here that used to stab buffet, even in level flight, no matter the power setting. Putting flap gap seals on stopped the shaking.
 
I have the flap ext. inboard to the fuselage and have never had a buffet problem.

Cubus Maximus
Are you talking about the gap seal on the top trailing edge over the fflap or a full fabric type gap seal. I have used the full fabric gap seal before and I prefer without it. Never had a buffet either way.
 
Christian,

I've heard/read that the flap gap seals make a big difference.

I'm doing extended flaps 4" over on mine. I was trying to figure out how to do the extended flap gap seal. I found Univair sells a 65" blank for $34.65/each vice the $43/each for the regular gap seals (with the hole already punched). You have to put your own flattened square hole in the blanks, which I trust wouldn't be a problem for you. Part number is -9212795-065

Good luck
 
Cal get your A*! out in that hanger and get to work on that plane (or are you still in Boston) :bad-words:
 
DW said:
I have the flap ext. inboard to the fuselage and have never had a buffet problem.
Click to expand...

Have you ever looked back at your tail on final approach with full flaps to see how much the stab is moving or shaking?

Maybe the flap gap seal is the cheapest place to start. They look better anyway.
 
I'm usually not looking back at the tail on final, but on runup--- OH YEA the tail shakes. I'm usually 41 to 45 on final.

PS with full flaps I can't see the tail.
 
I experience the same when landing. I never have felt it on any other cub besides mine. I'm curious if it is because I have metal flaps?

Tim
 
Anyone ever try to make a flap gap seal on the bottom, ala Cessna?:

182FGS.jpg


Not easy to do for standard PA-18 flap cove, but I have regular J-3 cove going all the way across. Bottom seal would be easier and probably more effective.
 
Bugs

I added flap gap seals to my Smith Cub after about 200 hrs of tail shaking when on approach with full flaps selected. I would say that 90% of the noticeable shaking went away. However, at 1100 hrs it was becoming noticeable again (not quite as much). I rebushed the outboard elevator hinges and also the rudder hinges.....shakin' went away!
Since your Cub is new I would say you need to add gap seals. I have the extended flaps (80") with 3 hinges, so we made them up from scratch.

Good luck

Lou
 
DW,

It's called a prosperity break. To interupt long periods of spend-aholism on cubs.

I'm in Raleigh. Back home tomorrow to the project. I'm not slacking at all. Still too busy to post pics....

Gap seals underneath sound cool, but I'd sure hate to mess with any flow through the gap between flap and wing. If it doesn't matter, which it may not, why did Piper put the strange angled flap contour on the supercub and the aileron cove contour on Pacers, etc?
 
lower is right, in my opinion. My C 170 has flap gap seals (were on there when I got it) and they are terrible. If it weren't for a lot of work to remove and make it look right, I'd take them off.

Consider that the Cessnas have semi Fowler flaps, and the Cub has slotted flaps. Why would you want to defeat the basic design of your flaps???

I've seen video of the stabilizer shake on Cubs by the fellow who developed and STC'd the BLR VG kit for the Cub. He experimented with a stock wing, then extended the flap inboard to the fuselage. With the stock wing, he demonstrated some buffeting of the horizontal stabilizer at high AOA and with flaps deployed full.

After extending the flaps inboard to very near the fuselage (ie: closing the gap between the inboard end of the flap and the fuselage) he demonstrated that this buffeting increased substantially.

This was all illustrated quite graphically with video of the tail (tufted) at high AOA.

Since the wing VG's permitted the wing to drive to a higher AOA, the buffeting was amplified even more than with the stock wing, hence the addition of the "strake" that is part of the BLR kit.

Those videos were pretty graphic.

If you're experimental, I'd consider building a strake similar to the BLR strake, and give that a try. It shouldn't be a big deal to make, and it virtually eliminates the buffeting of the stab.

MTV
 
Bugs66 said:
Anyone ever try to make a flap gap seal on the bottom, ala Cessna?:

182FGS.jpg


Not easy to do for standard PA-18 flap cove, but I have regular J-3 cove going all the way across. Bottom seal would be easier and probably more effective.
Click to expand...

I was told by a Cessna test pilot, at the time that these lower gap seals were installed on the 206, as shown in this picture, that the only difference was that the cruise speed increased by about the thickness of the line on the a/s indicator and that the stall speed increased by the same amount. The reason for the installation of the gap seals was only a marketing dept request.

In my opinion these would disturb the air flow through the slot that is created when the flaps are not up, thereby by reducing the effectiveness of the slot. The cruising speed of a Cub is too low for there to be much if any effect on the total drag.

Has anyone tried putting a fence on both ends of the flaps to help straighten the turbulent air that is generated when the flaps are down? I tried this on one flap on my 185 and found no difference at all. I did it on one flap to see if it would cause a roll in one direction during stalls. Nothing! Maybe on a Cub it will reduce the tail shake?
 
Thanks for the advice guys. I will start with a standard flap gap seal, then may experiment with strakes if buffeting still exists.
 
Lou,

The Smith Cub (180 with thrustline mod) that was here had the same issues until we we put gap seals on it. Shake went away.

Brad
 
I am with MTV - sealing the gap reduces the effectiveness of the flaps. I bet it would make a giant difference on something with real Fowler flaps, like a 737 or something.

How is it on the C model Husky? Those are closing in on a good flap design.
 
Bob,

Not sure what your question is regarding the C model Husky. It doesn't have gap seals on the flaps. The flaps on the C are same design as the earlier Huskys, except longer span.

They are effective, however.

MTV
 
flap gap

I have a smith cub, the flaps when down are only about 1/4" from the window, so far no tail shake but i do have the strakes on mine.For those that have the flap gap seal did it increase you climb or cruise speed?
Thanks
 
I just got back from flying with some flap seals that I made. It made a huge difference and my buffet all but disappeared. I guess Piper put them there for a reason. I did some brief power on/off stall tests. Not much difference power off but power on I swear I gained (decreased stall speed) maybe 2-3mph before it breaks.
 
Christian,

How did you make them. Piper style? Or, just an extended edge off the top o' the wing?

Glad you fixed it.
 
Could you post some pics of your gap seals? I am a long way off, but we will probably have this problem.
 
Iflylower said:
Christian,

How did you make them. Piper style? Or, just an extended edge off the top o' the wing?

Glad you fixed it.
Click to expand...

I took photos and will have them up on my site soon. They are easy to make and are similar to Piper style. I used .020 5052 and some PVC pipe to roll them. At first i tried some .016 roof flashing and they actually worked quite nice but they cracked on the edges where there is a 180 degree bend (despite my best effort for bend radius). The 5052 ones bend superb and are coming out great.
 
mvivion said:
So, you're talking about the gap seals on the TOP of the wings, right?

MTV
Click to expand...

Yeah top side ala Piper. I am waiting for some hardware to finish installing my newly fabricated flap seals.
 
My fabricated flap seals installed. I have more photos on my web site in the "Finishing Up" gallery.

im000052.jpg
 
What type of fastener did you use? Are they pop rivets or screws? It is difficult to tell from the photo.
 
skywagon8a said:
What type of fastener did you use? Are they pop rivets or screws? It is difficult to tell from the photo.
Click to expand...

I used #6 PK so I can remove if needed. My friends stock Cub has #4 but I went with #6.
 
Did the OP solve the problem? What needs to be done? Finding same results with a just finish -12 experimental.

Thanks!
larry
 
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